Zirconium is used for cladding a fuel rod in a nuclear reactor or an alloy with uranium and importantly used as an internal material in the construction of the nuclear reactor, or the like, due to its own various properties such as high neutron transmittance, corrosion resistance, or the like. Zirconium has high strength at high temperature and is not easily corroded by circulating coolant. In addition, zirconium does not easily form a radioactive isotope and is less mechanically damaged by irradiation of neutrons. Since hafnium (Hf) chemically similar to zirconium and contained in all of zirconium ore has large thermal neutron absorption, in order to use zirconium in the nuclear reactor, a content of hafnium should be controlled to be 100 ppm or less during a smelting process.
To this end, a solvent extraction method is used. In this method, Hf is separated from Zr using tributyl phosphate (TBP) as an extracting agent in ZrCl4 prepared by chlorination of the raw ore in an aqueous solution. The solvent extraction method may easily implement automation and have high separation efficiency to thereby be used in a wet refining process of commercial reactor-grade zirconium. However, as a raw material, chlorinated zirconium should be used, and a metal reduction process should be performed again through a conversion step since this process is a wet process, or the like, such that the process may be complicated.
After Hf is removed from Zr, a Kroll process is used at the time of preparing metal Zr. That is, precipitation and roasting are performed on Zr-salts subjected to solvent extraction to prepare ZrO2, and chlorination is performed again, thereby obtaining pure ZrCl4. This obtained ZrCl4 is reacted with magnesium at about 800° C. using vapor to form a Zr metal, and then fused in inert gas to form a Zr base metal. High-purity Zirconium metal (purity of 99.8% or more) processed as a plate, wire, or the like, may be obtained by thermal decomposition (pyrolysis) of zirconium iodide, molten salt electrolysis of zirconium chloride, or the like.
A smelting process of zirconium according to the related art has been disclosed in a number of references documents. For example, a Kroll process for preparing zirconium metal has been disclosed in U.S. Pat. No. 5,035,404, a technology of removing Hf using an ion exchange resin has been disclosed in Japanese Patent Laid-Open Publication No. 1998-204554, and a process of removing Hf from Zirconium by a solvent extraction method has been disclosed in U.S. Pat. No. 4,231,994.
A purification process for preparing high-purity Zr using iodine has been disclosed in Japanese Patent Laid-Open Publication No. 1991-501630, and an electrolytic refining process using low purity Zr was reported in the Journal of Electrochemical Society, Vol. 132, No. 5, pp. 1087˜1098 (1985).
Meanwhile, various pyrochemical processes for removing Hf from Zr using molten salts were arranged in Bulletin of Materials Science, Vol. 12, No. 3&4, pp. 407˜434 (1989).
However, in the technology disclosed in the above-mentioned existing processes, since ZrCl4 is prepared from zirconium raw ore by chlorination process and then a conversion process should be performed in multi-steps, chlorine gas having high toxicity is necessarily used, such that environmental pollution may be generated, and the process is complicated, such that there is a limitation in decreasing cost.
Therefore, the present applicant suggests a technology of performing direct metallization of zirconium from the raw ore without the chlorination process and recovering reactor-grade zirconium by an anode dissolution process.